This disclosure relates to a piston pump, especially a piston pump for pumping a liquid containing solid particles such as for pumping drilling mud in connection with recovery of hydrocarbons.
When pumping liquids containing solid particles the problem of wear arises due to the fact that the solid particles in the liquid become lodged between the piston and the cylinder. The wear occurs on both piston and cylinder after a period of use when the particles become lodged between the piston and the cylinder, resulting in the beginnings of a “scratch” in the cylinder wall. Once a scratch has started, larger particles will be deposited in the scratch and as time goes by there will be a leakage of drilling mud under high pressure. The piston must then be replaced. The cylinder is expected to have to be replaced after two piston replacements.
In order to reduce the wear, pumps can be equipped with a flushing system. This can include a pump with reservoir and a hose up to the rear of the piston where a casing with nozzles is installed. The nozzles spray water or oil, flushing cylinder and piston. However, there are still considerable expenses involved in replacing these parts relatively often.
From the prior art in the field we may mention U.S. Pat. Appl. No. 2004/0244577 A1 disclosing a piston pump which pumps liquid containing sand particles. The piston is provided with two seals 24 and 58. Between these packings an annular space 56 is formed which is supplied with a liquid through a bore 54 from a bore 36 passing centrally through the piston. The annulus 56 is pressurized thereby subjecting the seal 24 to the same pressure on both sides of the seal. Thus liquid will essentially not flow out of the annulus.
U.S. Pat. No. 4,598,630 discloses a double acting mud pump comprising an assembly of three pistons which are provided on a piston rod, a first (middle) piston and a second (left) piston and a third (right) piston which are provided movably in the axial direction on either side of the first piston. Between the third and the first piston a first chamber is created (C1) and between the second piston and the first piston a second chamber (C2) is created wherein the volume of the two chambers varies with the position of the second and the third pistons. A fluid is supplied to the chambers C1 and C2 through ducts in the piston rod. The pistons are provided with sealing elements which provide scalings between the piston and the cylinder.
U.S. Pat. No. 4,476,771 has the same applicant as U.S. Pat. No. 4,598,630, and discloses a very similar pump, but somewhat simpler in construction. The disclosed pump comprises a first (left) piston which is fixed on a piston rod and a second (right) piston which is attached axially movable on the piston rod relative to the first piston rod. This pump does not have any ducts at all for the supply of a fluid to a chamber (with a variable volume) which is created between the two pistons. The fluid is instead supplied by letting the fluid flow into the chamber between the second piston and the cylinder wall during the return stroke of the pump.
U.S. Pat. No. 4,270,440 discloses a drilling mud pump where a seal 74 with a gap filler 78 and a wear band 92 are mounted in the piston. These protrude further out than the rest of the piston, thereby forming an annulus 90 between the piston and the cylinder wall and the two seals. Water is supplied to this annulus through water channels 88 in the gap filler 78 and the channel 34. The wear band 92 is arranged with helically shaped ridges 94 and grooves 96, thus enabling the water to flow through the seal 92 and out at the rear of the piston in such a manner that, on account of the helically shaped grooves 96, the water obtains a rotating motion which flushes the cylinder wall when the water flows through the grooves 96 and out at the rear of the piston.
U.S. Pat. No. 3,720,140 discloses a drilling mud pump where the piston head is provided at its rear edge with a flange 15 which has a very small clearance to the cylinder wall. On the piston head, in front of the flange, a seal 20 is mounted to provide a seal against the cylinder wall. On the outer edge of the flange an annular groove is provided which during use can be supplied with a liquid under high pressure through a hose 31. According to the description the liquid is then forced through the small clearance between the flange and the cylinder, forming a thin film behind the piston. This film cools, cleans and lubricates during the piston's return stroke. Insofar as there is a flow of liquid, a thin film in this case, the liquid will substantially flow out behind the piston.
U.S. Pat. No. 2,367,185 discloses a drilling mud piston where the inside of the piston is equipped with an inner 35 and an outer ring 35′, and outside the inner ring 35 an inner wiping disk 39 and outside the outer ring 35′ an outer wiping disk 39′. Between the inner ring and the inner wiping disk there is an annulus which is supplied with a liquid through channels in the piston rod. Correspondingly, there is an annulus between the outer ring and the outer wiping disk 39′ which can be supplied with a liquid through channels in the piston rod. When the piston moves to the right, a negative pressure is created on the left side of the piston which draws water into the annulus (between inner ring 35 and inner wiping disk 39) and on into the cylinder (liner) 2 past the outer edge of the wiping disk 39. Correspondingly a negative pressure is created on the right side of the piston when it moves to the left, drawing water into the annulus (between outer ring 35′ and outer wiping disk 39′) and on into the cylinder past the outer edge of the wiping disk 39′.
Since there is no pressurized liquid in the annuli (the liquid is sucked into the return stroke by means of the negative pressure created) and the liquid which is sucked in can flow past the wiping disks 39, 39′, drilling mud will probably penetrate the annuli during the pump stroke. No mention is made of whether these wiping disks should be designed to permit liquid to flow past in one direction and prevent liquid from flowing past in the opposite direction. Nor is there any indication in the figures that the wiping disks are so designed.